An ink-jet printer includes a pen in which small droplets of ink are formed and ejected toward a printing medium. The pen is mounted to a reciprocating carriage in the printer. Such pens include printheads with orifice plates having very small nozzles through which ink droplets are ejected. Adjacent to the nozzles are ink chambers where ink is stored prior to ejection. Ink is delivered to the ink chambers through ink channels that are in fluid communication with an ink supply. The ink supply may be, for example, contained in a reservoir section of the pen or supplied to the pen from a remote site.
Ejection of an ink droplet through a nozzle may be accomplished by quickly heating a volume of ink within the adjacent ink chamber. The thermal process causes ink within the chamber to superheat and form a vapor bubble. Formation of a thermal ink-jet vapor bubble is known as "nucleation." The rapid expansion of ink vapor forces a drop of ink through the orifice. This process is called "firing." Ink in the chamber may be heated, for example, with a resistor that is responsive to a control signal. The resistor is aligned adjacent the nozzle.
Ink-jet printheads typically rely on capillary forces to draw ink through the ink channels to the ink chambers. As used herein, the term "back pressure" means a partial vacuum within the printhead. Back pressure is considered in the positive sense, so that an increase in back pressure represents an increase in the partial vacuum. The capillary forces overcome a slightly positive back pressure created by a regulator. Once ink is ejected from the chamber, the chamber is refilled by the capillary force, readying the system for firing another droplet.
As ink rushes in to refill an empty chamber, the inertia of the moving ink causes some of the ink to bulge out of the orifice. Because ink within the pen is generally kept at a slightly positive back pressure, the bulging portion of the ink immediately recoils back into the ink chamber. This reciprocating motion diminishes over a few cycles and eventually stops or damps out.
If a droplet is fired when the ink is bulging out the orifice, the ejected droplet will be dumbbell shaped and slow moving. Conversely, if the ink is ejected when ink is recoiling from the nozzle, the ejected droplet will be spear shaped and move undesirably fast. Between these two extremes, as the ink motion damps out in the chamber, well-formed drops are produced for optimum print quality.
Print speed (that is, the rate at which droplets are ejected) must be sufficiently slow to allow the ink motion within the chamber to damp out between droplet firing. The time period required for the ink motion to damp sufficiently may be referred to as the damping interval.
To lessen the print speed reduction attributable to the damping interval, ink chamber geometry has been manipulated. The chambers are constricted in a way that reduces the ink chamber refill speed in an effort to rapidly damp the bulging, refilling ink. Generally, chamber length and area are constructed to lessen the reciprocating motion of chamber refill ink (hence, lessen the damping interval). However, printheads have been unable to eliminate the damping interval. Thus, print speed must accommodate the damping interval, or print and image quality suffer.
Ink-jet printheads are also susceptible to ink "blowback" during droplet ejection. Blowback results when some ink in the chamber is forced back into the adjacent part of the channel upon firing. Blowback occurs because the chamber is in constant fluid communication with the channel, hence, upon firing, a large portion of ink within the chamber is not ejected from the printhead, but rather is blown back into the channel.
Blowback wastes some energy that is for ejection of droplets from the chamber ("turn on energy" or TOE) because only a portion of the entire volume of ink in the chamber is actually ejected. Thus, reducing blowback reduces TOE to increase the thermal efficiency of an ink-jet pen. Moreover, higher TOE results in excessive printhead heating. Excessive printhead heating generates bubbles from air dissolved in the ink, causing prenucleation of the ink vapor bubble. Air bubbles and prenucleation result in poor print quality.